With increasing enlargement of thin-film transistor (TFT) substrates, the size of reticles used in photolithography equipment for TFT fabrication is accordingly increasing, from the initial 6 inches to 920 mm×800 mm for 5.5th generation (G5.5) photolithography equipment and up to 1320 mm×1108 mm for 8.5th generation (G8.5) reticles. When such a huge reticle is retained on a reticle stage by suction, its downward bending due to gravity is inevitable, which can be as great as up to over 40 μm. For TFT-fabricating photolithography equipment of the 5.5th or more recent generations, the use of field-of-view (FOV) stitching or ultra-large FOV techniques has become an irresistible trend. However, the great impact of reticle warpage on exposure accuracy, in particular, on the focal-depth range of an objective, poses a great challenge to image quality guaranteeing.
One method for reticle warpage correction is to introduce an object-plane adjustment mechanism in the objective, which enables adaptation to gravity-caused reticle deformation. However, this leads to increases in structural complexity of the objective and the addition of a considerable number of movable members, which are not conducive to the reliability of photolithography equipment.
There is another method which uses a vacuum sealing technique in which a glass plate is mounted between the reticle and an illumination system such that the reticle, the reticle stage and the glass plate delimit a space which is configured to communicate with an air evacuation passage and an air introduction passage of a pneumatic device. A pressure in the space can be adjusted to produce an upward force equal to the reticle gravity, which bends the reticle in a direction opposite the direction of the gravity, thereby offsetting the gravitational bending of the reticle. However, the presence of the glass plate affects the performance of the illumination system in exposing the reticle. Moreover, the glass plate itself also experiences deformation caused by its own gravity, which further deteriorates the exposure performance. Further, the reticle, the glass plate and the pneumatic device are arranged on the same frame, which increases vibration and a weight load of the reticle.